Mitchell turned pro in 1981 and captured the WBA Junior Lightweight Title in 1986 with a 10th round KO over Alfredo Layne. He defended the title a record 12 times, and never lost a title fight. In 1991 he also captured the IBF Junior Lightweight Title with a decision win over Tony Lopez (who he'd previously fought to a draw). He retired after the fight, and came back briefly in 1994 for two local bouts, retiring completely in 1995 with a professional record of 45-1-3, avenging his loss and three draws in rematches and therefore being one of the few champions to beat everyone they faced.
Due to South Africa's apartheid policy during those times, the WBA ruled that Mitchell's title defenses were not to be held in South Africa. Mitchell then became a true 'road warrior', defending his title 12 times abroad.
Mitchell was a busy fighter who relentlessly came forward, throwing a high number of punches to his opponent's head and body. Many of Mitchell's defenses went the full distance of 15 rounds (7 defenses out of 12), but his points victories were usually unanimous and clear-cut. Mitchell always entered his fights in peak physical condition, and it was usually this dedication that earned him his victories.